This invention relates to an oxygenator such as an artificial lung utilizing porous hollow fibers.
In conducting open heart surgery for medical treatment of heart disease or of large blood vessels, it is customary to use an oxygenator. Oxygenators include a bubble type, a membrane type, etc. It is expected that a membrane type, which is smallest in damage done to the blood, will provide the majority of oxygenators in future. In an oxygenator of this type, a sheet of silicone rubber membrane is generally employed as a gas exchange membrane and the blood is passed along one surface of the silicone rubber membrane, with a stream of oxygen gas passed along the other surface of the membrane. In this case, oxygen in the stream of oxygen gas and CO.sub.2 in the blood are diffused across the membrane by the driving force created by the differences in partial pressure of oxygen and CO.sub.2 between both sides of the membrane, resulting in exchange of oxygen gas for CO.sub.2.
The conventional oxygenator outlined above is defective in that the silicone rubber membrane should be reasonably thick in view of the required mechanical strength thereof, leading to an inefficient CO.sub.2 removal from the blood. If the oxygen ventilation rate is increased in an attempt to improve the CO.sub.2 removal efficiency, the blood is caused to contain an excessive amount of oxygen. In other words, the respiratory quotient is caused to fall outside the physiological range.
It is also known in the art to use as the gas exchange membrane a microporous membrane made of a hydrophobic material in place of a silicone rubber membrane. In the conventional oxygenator, however, such a membrane is used in a plate or coil form, rendering it necessary to dispose a spacer between two adjacent membranes for preventing mutual blocking of the membranes and for keeping constant the passageways of the blood and gas. What should be noted is that the platelet is deposited on the spacer, resulting in failure of the treated blood to sufficiently recover its hemostatic ability. Further, thrombi are formed in the treated blood. Still further the spacer tends to cause damage to the membrane, causes pin hole occurrence and makes the artificial lung bulky.
It has also been proposed to use as a gas exchange membrane hollow fibers which permit smooth flow of blood and serve to eliminate a foreign matter in the blood passageway. Further, hollow fibers can be bundled, rendering it possible to assemble the device easily and to make the device smaller in size. It is one of the greatest requirements raised by the user to reduce the size of an oxygenator because a bulky device is difficult to handle. Still further, using a hollow fiber oxygenator facilitates removing air bubbles a in priming operation compared with using a plate or coil oxygenator. It is also important to note that the blood should flow through an oxygenator at a high rate, so that, the membrane housed in the device should be strong enough to withstand the pressure exerted by the blood. Needless to say, a hollow fiber is stronger than a flat membrane. An additional merit to be noted is that the device using hollow fibers is much simpler in structure than that using a flat or tubing membranes, resulting in that hollow fiber type devices are more uniform in quality. In spite of these merits, an aritifical lung utilizing hollow fibers has not yet been put to practical use because of the difficulties in selecting a suitable material for the hollow fiber and construction of the device permitting an improved O.sub.2 --CO.sub.2 exchange efficiency.
An object of this invention is to provide a hollow fiber type oxygenator having an improved gas exchange efficiency, and which is small in size, easy to handle and stronger than those using a flat membrane.